关于ruby：哪里是’Array＃sum`？

Where's `Array#sum`?

为什么[].sum给出了一个未定义的方法错误？

1 2	[5, 15, 10].sum # => NoMethodError: undefined method `sum' for [5, 15, 10]:Array

执行ri Array#sum返回：

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Array#sum

(from gem activesupport-4.2.6) Implementation from Enumerable
------------------------------------------------------------------------------
sum(identity = 0, &block)

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Calculates a sum from the elements.

payments.sum { |p| p.price * p.tax_rate } payments.sum(&:price)

The latter is a shortcut for:

payments.inject(0) { |sum, p| sum + p.price }

It can also calculate the sum without the use of a block.

[5, 15, 10].sum # => 30 ## <-- What?! >:(
['foo', 'bar'].sum # =>"foobar"
[[1, 2], [3, 1, 5]].sum => [1, 2, 3, 1, 5]

The default sum of an empty list is zero. You can override this default:

[].sum(Payment.new(0)) { |i| i.amount } # => Payment.new(0)

号

发生什么事？我为什么不明白？或者是我的安装坏了吗？